Schmitt triggers are commonly utilized in ICs to convert analog input signals to a digital output signals, where each Schmitt trigger generates a first (e.g., relatively high voltage) digital output signal value when a received analog input signal's voltage exceeds a higher trigger switching voltage value, and switches to a low output signal value when the analog input signal's voltage drops to (falls below) a lower trigger voltage value. Each Schmitt trigger has an associated hysteresis defined by the particular high and low trigger voltage values at which the Schmitt trigger switches between high and low output values (or low and high output values, depending on the Schmitt trigger's configuration). A problem with conventional (e.g., six-transistor) Schmitt trigger circuits is that the higher/lower switching voltage values can vary due to fabrication process variations, whereby conventional Schmitt triggers exhibit unreliable (inaccurate) hysteresis characteristics. When accurate hysteresis is required, conventional Schmitt triggers are typically enhanced by applying positive feedback to the noninverting input of a comparator or differential amplifier, whereby the hysteresis is defined by two predetermined trigger values that are determined by a reference voltage value supplied to the comparator or differential amplifier. For example, in a non-inverting Schmitt trigger configuration, the output signal value is high voltage level (e.g., 3.3V) whenever an applied analog input signal is higher than the Schmitt trigger's higher trigger voltage (e.g., 2V), and the output signal value remains high until the input signal subsequently falls below the trigger's lower trigger voltage (e.g., 2V), at which point the output signal value switches to a low voltage level (e.g., 0V). In contrast, an inverting Schmitt trigger (or Schmitt trigger circuit) generates a low output signal value whenever the input rises above the higher trigger voltage value, and generates a high output whenever the input falls below the lower trigger voltage value.
Many advanced image or radiation sensors are integrated circuits (ICs) including pixels formed by semiconductor processing (e.g., Complimentary metal-oxide-semiconductor (CMOS)) techniques, with each pixel including a sensing element, a self-reset circuit and an analog counter. For example, in high dynamic range (HDR) image sensors, each pixel utilizes a self-reset circuit to reset a photodiode to a maximum charge value each time the photodiode is saturated (i.e., each time the photodiode's stored charge falls below a predetermined minimum charge value), and utilizes an analog counter to determine the amount of received light (pixel response) by counting the number of times the photodiode is saturated and reset during a predefined exposure period. Schmitt-trigger circuits are often used to implement the self-reset circuit in such HDR image sensors, and are also utilized in conjunction with analog counters in a similar manner in Geiger mode single-photon avalanche diodes (SPADs). In both instances, the trend toward advance sensor IC devices including larger arrays (i.e., hundreds of thousands or millions of pixels) increases the need for self-reset circuits and analog counters that are small and exhibit low power consumption.
Conventional Schmitt triggers present an obstacle to developing smaller pixels and larger pixel counts in advanced sensor IC devices. For example, because a Schmitt trigger is utilized in the self-reset circuit of each pixel in order to determine the amount of received light during each exposure period, every Schmitt trigger utilized on an advanced image sensor is necessarily required to exhibit accurate and uniform (i.e., substantially identical) hysteresis characteristics in order for the advanced image sensor to generate usable image data. That is, if the Schmitt triggers of two pixels have different hysteresis characteristics (e.g., one has high/low trigger voltages of 3.1V and 1.9V, respectively and the other has high/low trigger voltages of 2.9V and 1.1V), then the two pixels would undesirably generate different image data values (i.e., different reset count numbers) when subjected to the same received light intensity. Unfortunately, the comparators and differential amplifiers utilized by conventional Schmitt triggers to achieve accurate and uniform hysteresis characteristics require significantly large amounts of chip area, and typically utilize bias voltages to maintain uniform hysteresis characteristics that consume DC current, and thus DC power, at all times during operation. The resulting large size and power consumption of self-reset circuits that utilize conventional Schmitt triggers enhanced with a comparator or differential amplifier becomes unacceptable when, as in advanced image and radiation sensors circuits, hundreds of thousands or millions of such counters (pixels) are required on a single chip. As such, the comparators or differential amplifiers required by conventional Schmitt triggers to achieve sufficiently accurate and uniform hysteresis present an obstacle to developing advanced sensor IC devices with higher density pixels and/or larger pixel counts.
What is needed is a Schmitt trigger circuit that overcomes the above-mentioned deficiencies associated with conventional Schmitt trigger circuits. In particular, what is needed is a small Schmitt trigger circuit that achieves accurate and uniform hysteresis without requiring the use of large comparator or differential amplifier circuits, and without the need for a DC bias voltage. Even more particularly, what is needed is a small, reliable, low-power-consumption Schmitt trigger circuit that can be produced with minimal changes to an existing semiconductor (e.g., CMOS) process flow that has been developed for fabricating advanced sensor and other IC circuits.